Non-volatile memory refers to a memory which is capable of keeping data after power-off (that is, stored date cannot be lost after power-off). EEPROM (Electrically Erasable Programmable Read-Only Memory) and flash memory (Flash Memory) all belong to the non-volatile memory. The non-volatile memory includes a plurality of memory cells and stores information by changing the number of electrons in floating gate of the memory cell. In the process of programming, after the electrons are injected into the floating gate of the memory cell, the threshold voltage of the memory cell is increased and in this point the memory cell is in a programmed state; in the process of erasing, after the trapped electrons in the floating gate are removed, the threshold voltage of the memory cell is reduced and at this point the memory cell is in a erased state. After programming or erasing is completed, data are stored in the non-volatile memory. At the moment, the memory cells of the non-volatile memory generally include both program cell (Pgm cell) and erase cell (Erase cell), which have different threshold voltages.
Erasure operation of the non-volatile memory is performed by blocks, that is, the erasure operation is performed on memory cells within one target erasure area each time. However, since the target erasure area may include both Erase cells and Pgm cells which may have significantly different threshold voltages before the erasure operation, if the erasure operation is directly performed on the target erasure area, then it may be difficult to ensure a convergence of threshold voltages of memory cells within the target erasure area after the erasure operation is completed.
To solve the above problems, a general method at present is to add a pre-programming process before the erasure operation. That is, a programming operation is first performed on Erase cells in a target erasure area to program them into Pgm cells, such that threshold voltages of memory cells within the target erasure area are relatively convergent; after the pre-programming operation is completed, an erasure operation is performed on the target erasure area.
Through above method, threshold voltage distribution of the memory cells within the target erasure area may have a good convergence after erasure operation. However, above process may increase the execution time of entire erasure operation; in particular, if the target erasure area is in a full-erasure state (that is, the memory cells in the target erasure area are all Erase cells) before erasure operation, the pre-programming process still may program all the Erase cells within the target erasure area in to the Pgm cells and then perform the erasure operation, which causes unnecessary time expenditure and complex process.